This invention relates generally to the field of negative index of refraction materials, and more particularly, to materials whose index of refraction can be tuned over a broad range of negative and positive values.
Material containing an electric permittivity and magnetic permeability that are simultaneously negative for some frequency have a negative index of refraction, and have been called left handed material (LHM) or negative index material (NIM). V. G. Veselago, in “The electrodynamics of substances with simultaneously negative values of ε and μ”, Soviet Phys. Usp. 10, 509 (1968), described some characteristics of such a material, including a negative index of refraction and the ability to re-focus light passing through a thin slab composed of this material.
In the mid to late 1990s, John Pendry described some thin wires and split ring resonators (SRR) that paved the way to the fabrication of a meta-material that exhibited the negative index properties at microwave frequencies.
In J. B. Pendry, et. al., Phys. Rev. Lett., 76 4773 (1996), Pendry disclosed that by using an array of thin metal wires, the plasma frequency of a metal could be shifted predictably to microwave frequencies. In J. B. Pendry, et. al., IEEE Trans. Microw. Theory Techniques, 47 2075 (1999), Pendry disclosed that by using an array of non-magnetic coupled metallic split ring resonators (SSR), the permeability of a metamaterial could be made to have negative values. This was demonstrated in the microwave by Smith in D. Smith, et. al., Phys. Rev. Lett., 84 4184 (2000), and more recently in the 100 terahertz range by Linden, in S. Linden, et. al., Science, 306, 1351 (2004). In these structures, the size and spacing of the individual components comprising the metamaterial are assumed much smaller than the wavelength of the resonant frequency of operation. They are also fixed frequency structures.
Smith's NIM structure used split ring resonators and strip lines made of copper over circuit board material. Smith's NIM structure is functional only at a single narrow band frequency, but demonstrated that microwave radiation passing through the wedged shaped NIM was bent through a large negative angle obeying Snell's Law, n1 sin θ1=n2 sin θ2. In such negative index materials, since n2 is negative, sin θ2 is also negative, yielding a large change in angle.
C. G. Parazzoli, et. al., Phys. Rev. Lett., 90 107401 (2003) A. A. Houck, et. al., Phys. Rev. Lett., 90 137401 (2003), have added additional confirmation to the results of Pendry and Smith, and further demonstrate the properties of a negative index of refraction predicted by Veselago.
Intrinsically photoconductive materials such as gallium arsenide and silicon have been used as high frequency substrates, with roll offs of greater than 50-100 GHz for Si and 1 THz for GaAs, as described in P. Abele, et. al., IEEE MTT-S Digest, 1681 (2002), D. W. Van der Weild, Appl. Phys. Lett. 65, 881 (1994), and U. Bhattacharya, et. al, IEEE Microwave and Guided Wave Letters 5, 50 (1995). Photoconductive bridging of strip line waveguides and resonators on Si have been observed at frequencies as high as 15 GHz with as little as 1 milliwatt per square mm CW illumination at 870 nm, as described in Y. Horri and M. Tsutsumi, Asia Pacific Microwave Conf., 561 (1997). More recently, an IR-defined, photoconductive microwave bowtie antenna in Si exhibited turn-on characteristics at 0.1 watts/cm2 at 970 nm with metallic (copper) like behavior at 1 watt/cm2 CW illumination between 1-18 GHz, as described in D. Liu, et. al., IEEE Photon. Technol. Lett. Vol. 10, page 716 (1998).
Some negative index of refraction composite materials are described in commonly assigned U.S. patent application Ser. No. 11/279,460 to Rachford, the entire disclosure of which is incorporated by reference herein.